Detailed investigations on thermal mode instabilities in LMA Yb-doped fibers

We present our investigations on modal instabilities using a commercial LMA fiber with 25 µm core and 400 µm cladding. The transversal mode instability (TMI) threshold is measured for different seed wavelengths and bending diameters in a systematic way. Additionally, in-situ temperature measurements up to the TMI threshold allowed measuring the longitudinal resolved thermal load and, thereby, fixing parameters for simulations that match experimental values. In conclusion, we derive driving parameters for TMI and found that photodarkening has a negligible impact on TMI in this fiber.